The long-term goal of this research is to develop new technology for breast cancer screening and diagnosis: Breast Elemental Composition Imaging (BECI). BECI is based on established evidence that malignant and benign breast tissue have different elemental compositions. The techniques used in these previous studies could not be translated into a practical clinical screening and diagnostic protocol. BECI can be performed in vivo (potentially without some of the current limitations of mammography). The components required for this technique are commercially available. Specific Aims are 1) Assemble and develop the system for non-invasive elemental composition analysis; 2) Acquire elemental composition measurements from excised benign and malignant breast tissue; 3) Train a classifier to distinguish benign from malignant tissue based on these measurements. Study Design: BECI measures the elemental composition using inelastic scattering of a neutron beam to excite characteristic energy states in the nuclei of the elements. As the nuclei de-excite, gamma rays are emitted with energies that are characteristic of the elements. These gamma rays are detected and their energy spectrum provides a signature of the elemental composition of the illuminated breast. This spectrum is analyzed by a classifier to distinguish between benign and malignant based on the differences between the elemental composition. The classifier will be trained using spectra obtained from excised benign and malignant breast tissue samples. In practice, the neutron beam would illuminate the patient's breast (not an excised sample), the emitted gammas will be detected, and the already-trained classifier would indicate the likelihood of breast cancer for that patient. At the neutron energies proposed in this work, there will be essentially no difference in the system performance for ex vivo and in vivo breast tissue. Preliminary dose calculations indicate that there will be enough signal over background to deliver significantly lower dose to the patient than current screening mammography.